An ion source utilizes an electron gun to generate electrons, which ionize neutral atoms and molecules. There are various apparatus, such as a mass spectrometer, which require an ion source. In a mass spectrometer, the ion source is used to ionize a gas which consists of neutral particles so that the ions of the gas can be analyzed. The electron-ionization source which has been generally used for mass spectrometers is the Nier source. Referring to FIGS. 1 and 2 there is shown diagrams of two views of a typical Nier source 10. The Nier source 10 comprises a chamber 12 having a front wall 14 and a back wall 16. A gas inlet passage 18 extends through the back wall 16 to allow a gas to be ionized to enter the chamber 12. An exit slit 20 is in the front wall 14 of the chamber 12. An electron source filament 22 is in the chamber adjacent the side wall 24 of the chamber 12. The filament 20 is connected to a source of electricity (not shown) for heating the filament 20. An accelerator plate 26 is adjacent but spaced from the outer surface of the front wall 14 of the chamber 12. The accelerator plate 26 has a slit 28 therethrough which is aligned with the exit slit 20 in the front wall 14 of the chamber 12.
In the operation of the Nier ion source 10, a gas of the sample material passes through the gas inlet passage 18 into the chamber 12. The filament 20 is heated so as to emit electrons. The electrons from the filament 20 pass transversely across the chamber 12 and across the flow of the gas. The electrons bombard the gas so that ions are generated by the electron impact. These ions are drawn through the exit slit 20 in the front wall 14 of the chamber 12 by the accelerator plate 26. The ions then pass through the slit 28 in the accelerator plate 26 and flow into a mass spectrometer (not shown). In the Nier ion source 10, ionization takes place only in the area of the gas which is traversed by the electron beam so that the amount of ions generated is relatively small. Also, the ions are generated only over an area defined by the length and width of the exit slit 20. These ions must then be focused onto the entrance of a quadrupole mass spectrometer, which is typically a small circular aperture. Therefore, additional focusing plates are required to ensure that all of the ions generated enter the mass spectrometer.